t.i.i.IHalfCloseableProtocol(Interface) : interface documentation

TCP supports the notion of half-closing the connection, e.g. closing the
write side but still not stopping reading. A protocol that implements this
interface will be notified of such events, instead of having connectionLost
called.

This indicates peer did half-close of write side. It is now the
responsibility of the this protocol to call loseConnection(). In addition,
the protocol MUST make sure a reference to it still exists (i.e. by doing a
callLater with one of its methods, etc.) as the reactor will only have a
reference to it if it is writing.

If the protocol does not do so, it might get garbage collected without
the connectionLost method ever being called.